1. Field of the Invention
The present invention relates to a method of manufacturing a solid electrolytic capacitor, and particularly to a method of manufacturing a solid electrolytic capacitor having a conductive polymer layer, with the use of a dispersion.
2. Description of the Related Art
As electronic devices have been digitized and adapted to higher frequencies in recent years, a capacitor having a small size, a large capacity, and low impedance even in a high-frequency domain has been required. A solid electrolytic capacitor having a conductive polymer layer has been developed as a capacitor meeting this requirement. Examples of solid electrolytic capacitors include a wound-type solid electrolytic capacitor and a stack-type solid electrolytic capacitor. For example, Japanese Patent Laying-Open No. 2007-294495 discloses a stack-type solid electrolytic capacitor including a metal plate.
For the conductive polymer layer, highly conductive polypyrrole, polythiophene, polyfuran, polyaniline, and the like are employed. Such a conductive polymer layer can be formed, for example, by impregnating an anode element with a polymerization liquid containing a precursor monomer of a conductive polymer, an oxidizing agent, and a dopant and causing chemical polymerization reaction.
In the conductive polymer layer formed through chemical polymerization reaction as described above, excessive oxidizing agent or unreacted precursor monomer may remain. In this case, characteristics of the solid electrolytic capacitor may disadvantageously be lowered due to a behavior of the remaining oxidizing agent or precursor monomer. It is noted that a higher capacitance, a lower equivalent series resistance (ESR) value, or a lower leakage current (LC) value can indicate high characteristics of the solid electrolytic capacitor.
In addressing the problem above, for example, Japanese Patent Laying-Open No. 2000-106329 discloses a method of washing the conductive polymer layer with a wash such as ethanol in order to remove the excessive oxidizing agent remaining in the conductive polymer layer of the solid electrolytic capacitor. In addition, Japanese Patent Laying-Open No. 2008-251629 discloses a method of washing the conductive polymer layer with a wash solution containing a solute in order to remove the unreacted precursor monomer remaining in the conductive polymer layer of the solid electrolytic capacitor.
In the method of forming the conductive polymer layer by causing chemical polymerization reaction on the anode element as described above, polymerization reaction gradually proceeds in a polymerization liquid from the time point of preparation of the polymerization liquid. Therefore, much attention had to be paid to handling of the polymerization liquid. In addition, since chemical polymerization reaction occurs on the anode element, a dielectric film on a surface of the anode element may be damaged.
In order to address this, a method of forming a conductive polymer layer with a dispersion has recently been developed. A dispersion is a substance obtained by dispersing a conductive solid composed of a polymer in a state of particle or aggregate in a solvent, and for example, a conductive polymer layer can be formed on an anode element by immersing the anode element in the dispersion so as to bring the conductive solid and the anode element in physical contact with each other. For example, Japanese National Patent Publication No. 2004-532298 discloses a dispersion in which polythiophene is dispersed. According to this method, no chemical polymerization reaction occurs on the dielectric film, and hence the dielectric film is not damaged. In addition, since no polymerization reaction in the dispersion proceeds as in the case of the polymerization liquid described above, handling thereof is easy.
It is difficult, however, to achieve a uniform size of particles or aggregates of a conductive polymer contained in the dispersion, and hence large particles or aggregates are present in the dispersion in many cases. In this case, since a large particle cannot enter the anode element, which is a porous body, and evenness of the conductive polymer layer is lowered. Consequently, characteristics of the solid electrolytic capacitor are disadvantageously lowered.